Preparation and operation of platinum plating baths



Patented Apr. 25, 1933 UNITED STATES PATENT, OFFICE ALAN RICHARD POWELL, EMYR GONWY DAVIES, AND ARTHUR WILLIAM SCOTT, OF LONDON, ENGLAND, ASSIGNORS TO JOHNSON MATTHEY & COMPANY LIMITED, OF

LONDON, ENGLAND PREPARATION AND OPERATION OF PLATINUM PLATING- BATHS No Drawing. Application filed January 23, 1932, Serial No. 588,492, and in Great Britain October 14, 1931.

This invention relates to the manufacture of a platinum salt for use in the preparation or replenishment of plating baths from which platinum may be electrolytically deposited in the form of a bright plate.

It has already been proposed to deposit platinum from baths consisting of a solution of platinum hydroxide (heXahydroxy-platinic acid) in caustic alkali solution and in the co-pending application of even date herewith a method is described by which this type of bath may be stabilized so as to produce bright deposits of platinum over long periods. As it is necessary to work with insoluble anodes, the bath must be replenished from time to time with a soluble platinum salt preferably an alkali metal salt of a platinic acid, as for example an alkali metal hexahydroxyplatinate. This salt is usually prepared by boiling platinic chloride, chloroplatinic acid, or a salt or chloroplatinic acid with an alkali metal hydroxide in large excess until crystals begin to separate; on cooling, the salt crystallizes in sandy-yellow crystals which tend to agglomerate into relatively hard crusts. For example, when sodium hexahydroxyplatinate, Na Pt(OH) is prepared in this manner it dissolves only with difficulty in water giving a more or less opalescent solution which gradually becomes more colloidal and finally yields a fiocculen't precipitate. Hence platinum plating baths prepared with this salt are not stable for long periods, require frequent filtration and yield dull or patchy deposits after a time.

The principal object of the present invention is to provide a finely divided dry crystalline platinum salt which dissolves readily in water without turbidity and which may be used for preparing and replenishing plating baths. Further objects of this invention are to provide a stabilized platinum plating bath from which bright plates of platinum may be obtained of almost any desired commercial thickness, to provide means for the replenishment of the bath, to provide suitable insoluble anodes, and to provide a suitable inert tank in which the plating operation may be carried out without the introduction of impurities into the bath.

\Ve have found that a readily soluble salt which yields stable plating baths from which bright depositsof platinum can be produced until the platinum content of the bath falls to from 0.4 to 0.5 per cent is obtained by treat ing a cold dilute solution of a compound of tetravalent platinum, as for example an alkali metal hexahydroxyplatinate, with an alcohol, ketone or other organic solvent which is completely, or almost completely, miscible with water. The organic solvent, as examples of which ethyl alcohol and acetone may be mentioned in particular, precipitates the platinum salt in the form of a pale yellowish crystalline powder containing more or less water of crystallization depending on the or- ,ganic solvent used for its precipitation and the conditions under which it is precipitated. Platinum salts prepared in this manner dissolve freely in cold or warm water giving clear, stable solutions.

The following example will further illustrate how the invention may be carried out in practice, but the invention is not restricted to this example.

E mample 150 ounces of sodium chloroplatinate (Na Ptcl fil-LO) are dissolved in 2 gallons of hot water and the solution is added to 3 gallons of hot water containing 15 ounces of sodium hydroxide. 135 ounces of sodium hydroxide in solid form are then added to the hot solution. The solution boils vigorously so that cold water must be added as required to keep the boiling under control. When all the sodium hydroxide has dissolved the solution is diluted to 15 gallons with hot water and boiling is continued for from 3 to 4 hours, until the colour of the liquid changes from orange to a pale yellow. The solution is then cooled to 20 centigrade, and from 10 to 15 gallons of commercial ethyl alcohol are added while stirring constantly, whereby a finely crystalline precipitate of the sodium hexahydroxyplatinate is obtained. After the precipitate has settled the clear liquor is siphoned off and the crystals collected on a vacuum filter, washed with a mixture of equal volumes of alcohol and water, then with alco- 1101 alone and finally with acetone. The crystals are then dried in a current of air at a temperature preferably not exceeding 40 centigrade, until a fine crystalline powder is obtained. Overheating must be avoided as the organic liquid reduces the salt to platinum black at high temperatures. Under the conditions specified about 88 ounces of the salt are obtained containing 54.4 per cent of platinum and representing about 94 per cent of the platinum used in the manufacture. The corresponding potassium or other alkali metal salt is obtained in a similar manner using potassium or other alkali metal hydroxide and potassium or other alkali metal chloroplatinate in place of the corresponding sodium compounds.

In preparing a plating bath from, for example, the sodium salt, we may proceed as follows :27 ounces of the salt are dissolved in 7.5 gallons of a warm solution containing- 7.5 ounces of pure sodium hydroxide, and a hot solution of 45 ounces of sodium sulphate (to act as a conducting salt) is then added and the mixture diluted to 10 gallons. This gives a bath containing approximately 1 per cent of platinum. This strength of bath is sufiicient for most purposes, but stronger solutions can be readily prepared and work just as well, or even better, especially at high current densities. \Ve have found that the operation of the bath is considerably improved by adding small quantities of stabilizing salts such as alkali metal oxalates; this action may be due to the formation of complexes which tend to prevent or retard the hydrolysis of the platinum salt.

In the above example of a plating hath made up according to this invention it will be seen that the free alkalinity corresponds with a content of 0.5 per cent of sodium hydroxide. As the platinum is removed from the electrolyte by plating this free alkalinity will increase. We have found that it is important, if bright deposits are required, to limit this increase by occasional neutralization of the undesirable excess of sodium hydroxide by the addition of sulphuric acid; this is most conveniently effected when the bath requires replenishment with platinum as hereinafter described. While we do not limit ourselves to any definite degree of alkalinity we prefer to operate with an alkalinity corresponding with a sodium hydroxide content of less than 2.5 per cent; with a greater alkalinity matt deposits are usually obtained; We therefore prefer to work with an alkalinity corresponding with a sodium hydroxide content not exceeding 1 per cent advantageously between 0.2 and 1 per cent.

Suitable insoluble anodes for use in the baths prepared according to this invention comprise precious metals or alloys which are not attacked by the electrolyte and are not oxidized by the nascent oxygen generated at the anode. Carbon anodes are rapidly disintegrated and cause fouling of the bath, and base metals are all more or less readily attacked.

Gold and the metals of the platinum group or alloys of these alone are satisfactory in all respects, but are relatively costly.

We have now found that relatively inexpensive and satisfactory anodes are produced by coating base metal anodes with a suitable sheathing of gold, metals of the platinum group or alloys of these metals. We have obtained excellent results from copper sheet anodes plated with platinum according to this invention. Gold plated anodes may also be used.

The baths made up in accordance with this invention give good plates of platinum when operated at temperatures above 40 centigrade at current densities up to 20 amperes per square foot, until the platinum content falls to about 0.4 per cent. To replenish the bath with platinum we prefer first to add 1.8 ounces of the salt for every 1 ounce of platinum which is required to bring the concentration of platinum in the bath up to the desired degree, then to neutralize the free sodium hydroxide which has been liberated from the sodium hexahydroxyplatinate; this is effected by adding, in the form of a dilute solution, 0.5 ounce of sulphuric acid for every 1 ounce of platinum which has been deposited.

In operating plating baths made up according to this invention we find that there is a tendency for silica to accumulate in the bath when glass, stoneware, earthenware, silica or similar vessels are employed. This aecun'ullation of silica leads sooner or later to the production in the bath of a flocculent precipitate consisting of gelatinous silica flocculated with a small quantity of a platinum compound. lVhile good plates can still generally be obtained from baths containing silica, there is a tendency for defects to occur in the plates produced from old baths containing relatively large amounts of silica and occasional filtration is therefore necessary.

\Ve have now found that these difiiculties may be avoided by using tanks constructed of, or lined with, a metal which is not attacked or corroded by the electrolyte. Such metals comprise silver, nickel and alloys with a high content of these metals, as for example, silver copper, nickel-copper and nickel-chromium alloys; passivated iron alloys such as the stainless steels, as for example that known under the registered trademark Staybrite and the like are also suitable for use as tanks. We have obtained excellent results with tanks made of pure nickel. In the case of tanks lined with a resistant pure metal, a satisfactory lining may often be produced by electro-plating the interior.

For example, copper tanks may be plated internally with silver, nickel, old or platinum of a sufiicient thickness to prevent chemical attack on the copper.

T o prevent contamination of the platinum plating salt with silica compounds during manufacture, we prefer to carry out the manufacture in metal vessels of the type hereinbefore mentioned; pure nickel or heavily nickel-plated copper or iron-vessels are especially suitable for this purpose.

Vhat we claim is 1. A process for the manufacture and production of adherent deposits of platinum by electrolysis in which the plating bath comprises an alkaline solution containing a conducting salt, and a readily soluble-crystalline alkali metal platinate prepared by treating the reaction product of an alkali metal hydroxide and a compound of tetravalent platinum in aqueous solution with an organic liquid which reduces the solubility of the alkali platinate.

2. In the process as claimed in claim 1, adding a stabilizing salt, as for example an alkali metal oxalate to the bath, to retard or prevent the hydrolysis of the platinum salt.

3. In the process as claimed in clalm 1, maintaining the free alkalinity of the plating bath below that corre ponding with a sodium hydroxide content of 2.5 per cent.

4. In the process as claimed in claim 1, maintaining the free alkalinity of the plating bath at a value corresponding with a sodium hydroxide content between 0.2 to 1 per cent.

5. In the process as claimed in claim 1 maintaining the plating bath used relatively free from silica or compounds thereof.

6. In the process as claimed in claim 1, the use of an electrolytic tank constructed of a metal which resists the action of the electrolyte, as for example, silver, nickel, or alloys thereof, or stainless steels.

7. In the process as claimed in claim 1, the use of an electrolytic tank constructed of a non-resistant metal which is plated with a resistant metal, as for example silver, nickel, gold or platinum.

8. In the process as claimed in cla1m 1 employing a base-metal anode plated with gold, platinum or alloys thereof.

9. In the process as claimed in clalm 1, employing as the platinum salt sodium hexahydroxyplatinate precipitated by the addition of an organic solvent completely or almost completely miscible with water to a solution obtained by boiling a platinic compound with sodium hydroxide solution.

10. In the process as claimed in claim 1, employing as the platinum salt potassium hexahydroxyplatinate precipitated by the addition of an organic solvent completely or almost completely miscible with Water to a solution obtained by boiling a platinic compound with potassium hydroxide solution.

a solution obtained by boiling the platinic compound with an alkali metal hydroxide solution, and employing ethyl alcohol as the precipitant.

12. In the process as claimed in claim 1, employing as the platinum salt an alkali metal hexahydroxyplatinate precipitated by the addition of an organic solvent completely or almost completely miscible with water to a solution obtained by boiling the platinic compound with an alkali metal hydroxide solution, and employing acetone as the precipitant. a

13. The process for replenishing plating baths prepared according to claim 1 which consists in adding thereto an alkali metal 'hexahydroxyplatinate prepared by precipitation by the addition of an organic solvent completely or almost completely miscible with water to a solution obtained by boiling a platinic compound with an alkali metal hydroxide solution.

14. In the replenishment of plating baths prepared according to claim 1, which consists in adding thereto an alkali metal hexahydroxyplatinate prepared by precipitation by the addition of organic solvent completely or almost completely miscible with water to a solution obtained by boiling a platinic compound with an alkali metal hydroxide solution, neutralizing part of the free alkalinity by means of sulphuric acid before or after addition of the platinum salt.

In testimony whereof we have signed our names to this specification at London, England, this sixth day of January, 1932.

ALAN RICHARD POWELL. EMYR CONWY DAVIES. ARTHUR WILLIAM SCOTT. 

